Apparatus and method for digesting



July 29, 1958 H. J. BYRNE 3 3 r APPARATUS AND METHOD FOR DIGESTING FiledMa 18, 1955 3' Sheets-Sheet INVENTOR. HUGH J. BYRNE J ch &

ATTORNEY July 29, 1958 H. J. BYRNE APPARATUS AND METHOD FOR DIGESTINGFiled May 18. 1953 3 Sheets-Sheet 2 INVENTOR.

HUGH J. BYRNE A'TTORNEY July 29, 1958 H. J. BYRNE 3 APPARATUS AND METHODFOR DIGESTING 3 Sheets-Sheet :5

Filed May 18, 1955 INVENTORL HUGH J. BYRNE 3 1 HE Mu( W ATTORNEY UnitedStates Patent 2,845,347 APPARATUS METHOD FOR DIGESTING Hugh J. Byrne,Seattle, Wasl.

Application May 18, 1953, Serial No. '355,662

20 Claims. (Cl. 92-7) My present invention relates to apparatus andmethod for digesting materials, particularly fibrous materials such aswood chips used in the manufacture of pulp and paper products. Theinvention is of utility in the formation of any fibrous material by thedigestion of a raw material such as woo d chips, straw, stalks offibrous plants, or the like. The present invention comprises, inessence, an improvement upon standard apparatus and conventional methodsof digesting materials, such as dsclosed in the patents to Christensen,1,942,685, and Richmond, 2,163,437, for example, being concernedparticularly with the problem of preventing the screens within thedigester from becoming clogged.

The present invention achieves its highest utility in the wood pulpindustries, and may be utilized in the digesting of wood pulp by anyChemical method, such as the sulfite process (arrmionia base, magnesiumbase, calcium base, etc.), and the sulfate or Kraft process, sodaprocess, semi-chemical process, etc. In such processes a large pressurevessel is charged with a quantity of wood chips or other fibrous rawmaterials, and subjected to the digesting action of a heated liquor,acidic or basic, as the case may be, under pressure, until the fibrousmaterial is broken down into individual fibers or small bundles offibers as desired, and Chemical Components which may remain in partadhered to the fibers or bound therein, and in part are dissolved in orSuspended in the cooking liquor. The fiber is then subjected to furthertreatment by known processes and formed into paper, pulp, or otherfibrous articles.

Widely varyng circulating systems are associated with the digesters inan eflort to achieve uniformity of digestion. It is quite common tocirculate the cooking liquor from the digester through a heat exchangerand back to the digester, as dsclosed in the Christensen patent. Inorder to prevent the escape of fibrous material from the digester whenthe cooking liquor is withdrawn from the digester by means of a pump,the suction inlet to the pump is protected by a screen such as dsclosedin the Christensen and Richmond patents mentioned above. Since thecooking liquor is Corrosive the digester shells are sometimes lined witha protective lining such as ceramic, or stainless steel. The reheatedliquor may be returned to the digester at different points in an efortto prevent overheating and charring, or undercooking of 'the fibrousmaterial at various levels in the digester. Instead of using a heatexchanger in the liquor circulating system heat may be added byintroducing 'steam to the digester at different points.

When external shell and tube heaters are not used, a common arrangementis to provide the suction inlet screen for the liquor at a higher levelin the digester and to introduce steam at the bottom of the digester,and to return the liquor at either the top or the bottom or both, in anattempt to achieve substantial uniformi ty of cooking action. Usua lly,in such cases, live steam is admitted at the bottom of the digester andthe cooking ice liquor is returned at the top of the digester. This ararangement is subject to several disadvantages, -as follows:

First, there is a tendency to overcook the lower layers of fibrousmaterial and to undercook the upper layers;

second, the steam is `absorbed by the liquor while passing.

upwardly through the fibrous material, causing imperfect digestion dueto the liquor becoming more nearly neutral in the lower portion of thedigester; and third, the practice often results in the 'Creation ofwater hammer which may tear loose the protectve lining, or cause othermechanical failure. An object of the present invention is to providemeans for recirculating the cooking liquor, with or without externalheating thereof, while controlling the heat and shortening the cookingtime in a manner to prevent unequal digestion, overheating orundercooking in different areas, and water hammer.

A further object of the present invention is to lessen the cost ofinstallation and upkeep of the suction screen for the liquor circulatingsystem in a digester of the' character described. In some digesters awhole or par-- tial ring of stainless steel screens has been Situatedaround the interior of the digester shell at an intermediate level'.

through which cooking liquor is withdrawn from the di-- gester, thescreens usually being perforated by a plu-- rality of small holes in theorder of one-eighth to threeeighths of an inch in diameter. Theperforations in the: screen must be small enough that they do notpermit' the passage of more than a limited amount of separated fibers,but must be large enough that they do not clog: easily. The resultingcompromise is such that a plu-- rality of fibers and small pieces of theraw material are: attracted to the screen and build up bridges over thescreen openings, or otherwise plug them, the formation progressing untila mass of interlaced, fibrous material plastered onto the front of thescreen prevents removal of cooking liquor through it. As a resultcirculation is dirninished and later stops and cooking must continuewithout circulation, the resulting pulp being of poorer quality thandesired.. The diminishing flow of cooking liquors through thecirculating system results in prolongation of the cooking time andincreases the likelihood of uneven cooking throughout the digester. Afurther object of the present invention is to provide means foragitating the accumulated mass of fibers in front of the inner side ofthe digester screen whereby clogging of the screen is prevented so thata constant flow of cooking liquor through the screen into thecirculating system is achieved, thus improving uniformity of digestionthroughout the digester `and diminishing the average length of cookingtime.

A further object of the present invention is to provide means whereby ascreen of less overallsurface area, or one having a fewer number ofperforations, or a plurality of spaced, small screens, may besubstituted for the present annular ring of closely perforated screenscompletely surrounding the inten'or of the digester shell, thus reducingthe cost of installation and maintenance.

A further object of' the present invention is to provide means forintroducing a fluid in proximity to the inner surface of the circulatingsystem screen to achieve the dual purpose of maintaining adequatecirculation and controlling the character of the cooking liquor. Thefluid introduoed may bea portion of the cooking liquor, an nert gas such'as nitrogen, air, steam, or an active control gas or other fluid suchas carbon dioxide -for acidic cooking liquors or ammonia for causticcooking liquors. Thus the present invention permits a large choice ofintroduced fluid for the purpose of fluffing oragitating the mass offiber adjacent to the screen, which otherwise could block its apertures,and controlling the strength or character of the cooking liquor, whileintroducing the fluid at the most advantageous point to achieveuniformity of digestion and eliminate water hammer.

A further object of present invention is to provide a method ofachieving uniformity of digestion and shortening the time of cooking ina digester by introducing a fluid under pressure at a plurality ofpoints around the interior of the digester at an intermediate level.

A further object of the present invention is to provide a method forcontrolling the character and quality, such as the pH value of digestingliquor, by introducing controlled amounts of an active control fluidduring the process of digeston.

Further objects of the present invention are to reduce the temperaturerise period in a digester and thereby reduce the' overall time cycle; toprolong the time that adequate circulation can be maintained, and thusreduce the overall time cycle and produce more uniformly cooked pulp; toreduce the chemical demand by achieving uniformity of cooking over theentire cooking period; otherwise to improve the result and dirninish thecost of digesting pulp; and to reduce costs by eliminating the eX- penseof replacing condensate with boiler feed make-up water in systems whererecirculated liquor or gases are used for screen scavenging.

The foregoing and other objects and advantages of the present inventionmay be more thoroughly understood by reference to the accompanyingdrawings, taken in connection with the following specification.

Fig. l is a schematic representation of a digester and circulatingsystem including a heat exchanger, with a part of the digester shellbroken away to illustrate a form of the present invention.

Fig. 2 is an enlarged, vertical section taken radially of the digestersubstantially along line 2-2 of Fig. 1.

Fig. 3 is a partial vertical section along a chordal plane of thedigester showing a modification of the apparatus of the presentinvention.

Fig. 4 is a horizontal, schematic section through a digester having afurther modification of the apparatus of the present invention.

Fig. 5 is an enlarged detail view taken from the plane 5-5 of Fig. 4.

Fig. 6 is a horizontal section taken substantially along line 6- 6 ofFig. 5.

Fig. 7 is a schematic representation of a modified form of digester andcirculating system embodying the present invention.

Fig. 8 is a schematic representation of a further modified form ofdigester and circulating system embodying the present invention.

Fig. 9 is a partial vertical section substantially corresponding to Fig.2 illustrating a further modification of a portion of the apparatus.

In Fig. l I have illustrated a digester 10 which comprises a cylindricalshell having a rounded dome 11 and a conical bottom 12, representativeof a conventional digester. In accordance with conventional practice theshell of the digester is fabricated of structural 'steel so as towithstand pressures, and the interier thereof is lined with acorrosion-resistant lining of suitable nature. The digester is providedwith a filling neck 13 at the top which may be closed by a suitable lid14. It will be understood that the interior of the digester is filledwith appropriate raw material and the desi'ed cooking liquor.

A conventional circulating system is associated with the digester shell,including a suction screen ring 15 horizontally disposed within theshell in the form of an inwardly protruding, annular ring at anintermediate level within the shell. Cooking liquor is withdrawn throughthe screen and a suction pipe 16 having a valve 17 therein by means of apump 18. The pump discharges through a discharge line 19 having a checkvalve 20 and a valve 21 therein. The discharged liquor passes throughtubes in a shell and tube heat exchanger 22 having steam 4 admittedthereto through an entr'ance tube 23 having a valve 24 therein, thecondensate being withdrawn through a discharge tube 25 having a valve 26therein. The heated liquor passes from the heat exchanger into either orboth of two return lines, one line 27 leading to the top of the dome 11through a valve 28, and the other line 29 leading to the bottom of thecone 12 through a valve 30 and check valve 31. Admisson of liquor at thebottom of the cone is achieved through a T-fitting 32 forming a portionof a blow line 33 having a valve 34 therein. At the top of the domethere is provided a vent line 35 having a pressure relief valve 36therein for the purpose of withdrawing waste gases. The digester andcirculating system, as described, is conventional and representative ofsuch systems.

lt is common practice to form the screen ring 15 (Fig. l) of a pluralityof screen segments 40 made of stainless steel, each screen segmentcomprsing a stainless steel hacking plate 41 (Fig. 2) which may be heldagainst the lining 42 of the digester shell by means including clamps 43which are held in position by nuts 44 engaging studs 45 mounted on thedigester shell, or other suitable means. Each screen segment preferablycomprises a plate 46 perforated by suitable perforations 47, the platebeing held in outwardly bowed condition by internal ribs 48, which arewelded to the hacking plate 41. Individual segments engage each other attheir vertical edges so as to form a continuous screen around theinterier of the digester. Individual segments or the entire screen maybe replaced when excessively worn or damaged. The screens illustratedare representative of practical installations, but scavenging apparatusof this invention may be applied to screens of any shape or size.

In accordance with the present invention I am enabled to reduce theoutward bulge of the screen, keep the upper and lower legs thereof atacute inclinations to the wall to prevent the material from hanging upon the screen during blowoff, reduce the number of perforations, andreduce the number of screen segments. As illustrated in Fig. 1, thescreen ring may be continuous, comprising a plurality of abutted screensegments, but the fabrication thereof may be simplified and the costreduced by reducing the total screen area.

In accordance with the form of the invention illustrated in Fig. 1, Iachieve the objects of the invention by providing a scavenging ring 50adjacent to the surface of the screen ring which surface is convexaxially of such ring and concave circumferentially of that ring, thescavenging ring being perforated at suitable intervals whereby a fluidunder pressure may be introduced in a direction tangential to the screensurface and at a plurality of points, preferably, but not necessarily,in alignment with vertical rows of perforations to prevent clogging ofsuch screen ring perforations. A form of such scavenging ring isillustrated comprising a continuous ring 50 of one and one-quartcr inchtubing having upwardly directed three-sixtcenths inch diameter holes 51on two-inch centers in alignment with vertical rows of perforations 47.In a typical example the minimum inner diameter of the screen ring isforty-four and one-quarter inches, measured radially of diametricallyopposed screen segments, and the mean diameter of the scavenging ring isforty-five inches, so that the jet openings 51 therein, locatedsubstantially along vertical diameters of the scavenging ring. slightlyunderlie the innermost portion of the screen ring bulge. The scavengingpipe is, in this. form of the invention, located below the lowermost rowof perforations in the screen segments and its external surface isspaced from the adjacent surface of the screen ring by a distance ofapproximately three-quarters of an inch. Preferably the scavenging ringis made of two semicircular pipe sections 52 joined together by a pairof unions 53, in order to facilitate assembly. The scavenging ring issupported by a plurality of supporting arms 54, which may beConveniently attached to certain one's of the screen mounting clamps 43.The scavenging ring is connected to .a riser pipe 55 which is connectedby a pipe union 56 to the upwardly turned arm of an inlet pipe 57 havinga valve 58 therein.

A desired fluid, either steam, inert gas, digester liquor,`

or a control gas or other fluid which will combine with the cookingliquor, may be introduced continuously or intermittently under pressurethrough the scavenging ring. The jets or fluid under pressure sweepupwardly across the screen and fluffor dislodge the mat of fibers orchips formed in front of the perforations 47, thus constantly, or asoften as desired, clearng the perforations of the screen so that thecirculation may be maintained at a high rate. Regardless of what fluidis introduced at the intermediate level in the digester, greateruniformity is achieved due to the maintenance of a more uniformtemperature throughout the digester, as distinguished from priorpractices where, for example, steam was admitted at the bottom of thedigester. This eliminates condensation weakening the liquor at thebottom of the digester, greater circulation throughout the digester isachieved, and results in greater uniformity and less cooking time beingrequired.

In the form of the invention illustrated in Fig. 3, a digester having ascreen ring, asipreviously described, is provided with a scavenging ring60 located above the screen ring in substantially the same manner thatthe scavenging ring 50 is located below the screen ring in .Fig. 1, inthis instance the jet openings being directed downwardly across the faceof the digester screen.

In Figs. 4, 5 and 6 I have illustrated a modification of screen andscavenging inlet Construction, of less expense since a plurality ofwidely spaced screens may be used. In this modification a digester shell61 having a liner 62 is provided with a plurality of spaced screens,each comprising a mounting plate 63 and a perforated, frustropyramidalscreen 64 in which a plurality of `perforations 65 are, provided. Eachof the screens is suitably maintained circumferentially around theinterior of the shell by suitable clamps and the interior thereofcommunicates 'with a branch 66 of a circulating manifold 67 surroundingthe exterior of the shell, the manifold being connected to thecirculating pump inlet tube 68. Each mounting platel63 has weldedthereto or otherwise suitably aflixed thereto, a scavenging fluid inlettube 69 connected to a vertical jet head 70havng a plurality of jetoutlets 71, preferably, but not necessarily, in horizontal alignment,respectively, With rows of the screen perforations 65. The tubes 69 areconnected to an inlet manifold 72 with which communicates the fluidinlet line 73. r

As so far described the modifications favor the use of steam as ascavenging and heating medium or a gas for scavenging or chemicalcontrol purposes.

A modification of the system illustrated in Fig. 7 favors the use of aninert gas or an active control gas to keep the screens clean and tocontrol the character of the cooking liquor. In this modification adigester 80 is provided with a screen ring 81 adjacent to which is ascavenging ring 82, it being appreciated that the screen ring andscavenging ring are representative of the various previously describedmodifications. A gas vent line 83 is provided and liquor is circulatedthrough suction line 84, pump 85 and discharge line 86 having branchesleading to the top and bottom of the digester, suitable valves beingschematically illustrated. The shell and tube heat interchanger isomitted. The blow line 87 is provided with the usual valve 90 and abranch inlet line 88 for the introduction of heat controlling steam, ifdesired,

i having a valve 89. The scavenging ring is connected to a fluidadmission line 91 in which there is a valve 92. This system favors theuse of an inert gas such as nitrogen, or an active control gas such ascarbon dioxide for acid cooks, or ammonia for caustic cooks, in order tocontrol the acidty or caustcity (pH value) of the cooking liquor.

6 Steam may be used if desired in 'such instances, admitting all steamfor scavenging and heating purposes through scavenging ring 82 anddispensing with the conventional steam inlet line 88' of conventionalsystems. The control of cooking temperature may be achieved by preheatngthe gas ntroduced through the scavenging ring An alternative arrangementis disclosed in Fg. 8 which favors the use of the circulated liquor toscavenge the screen as it does not alter cooking liquor strength. Inthis figure a digester 93 is shown, having a screen ring 94, scavengingring 95 and the usual vent line 96 and blow line 97. To avoid clogging,the scavenging ring jets should be larger than the screen ringapertures, the former being 4 diameter andthe latter diameter forexample; The liquor is withdrawn through the screen ring and the suctonline 98 incorporating valve 99 by pump 100 from which it is 'dischargedthrough check valve 101, valve 102, shell and tube heater 103, and oneor any combination of a plurality of return lines 104, and 106 asdesired. The return line 104 leads to the top of the digester through avalve 107. The return line 105 leads to the scavenging ring 95 through avalve 108. The return line 106 leads to the bottom of the cone through avalve 109 and check valve 110. The vent line may be closedby theconventional valve 111 and the blow line may be closed by theconventional valve 112. The shell of the heater may be provided withsteam through an admission line 113 having a valve 114, the condensatebeing withdrawn through an exit line 115 and condensate trap 116. Forincreased pressure of the scavenging liquor, line 105 may be connecteddirectly to the outlet of pump 100 or may include a booster pump.

In Fig. 9 I have illustrated another modification of the screen andscavenging ring arrangement. In this installation the scavenging ringpipe is placed in the space between the hacking plate 131 and the screenring plate 132. The scavenging pipe has short branches 133` whichterminate at the adjacent surface of the screen plate 132,

-each surrounding one plate aperture 134. scavenging fluid projectedthrough such apertures by the branches 133 blasts the chips and pulptoward the center of the digester to clear apertures above and belowapertures 134 to enable liquor to flow out through apertures 135. Thescavenging ring thus located is protected from damage by pressure surgesin the digester.

I have illustrated a number of different types of circulating systems todemonstrate the versatility of the present invention in that varioustypes of fluid may be used in connection with a number of diflerenttypes of arrangement of screen and scavenging ring fluid inlet means. Insome systems it may be desirable to circulate a portion of the liquorthrough the scavenging ring rather than introducing steam, achieving allheating by use of an external heat exchanger from which condensate maybe salvaged, this being of extreme value in plants where boiler feedmakeup Water is expensive. A further point in favor of 'circulating aportion of the liquor through the scavenging ring rather thanintroducing steam is that ordinarily a digester is charged withsufiicient liquor at sufficient strength to complete the whole cookingoperation, with the result that the inital cooking of the externalsurfaces of the Wood chips or other material may be too drastic, whichdisadvantage may be eliminated by introducing a milder liquor at firstand then introducing a control gas or control lquid to keep up thestrength of the liquor or to increase its strength at a subsequentperiod during the cooking operation.

Further advantages of introducing a control gas or liquor are that theliquor strength is controlled and that dilution of the liquor as thecook progresses is prevented, as would be the case with the use ofsteam, and better quality of high grade pulp is achieved. On the otherhand, an inert gas, heated or unheated, has the same advantages asexternally heating the cooking liquor in a heat excha'nger and maybedesirable in some instances of one type of fluid over another,especially for the production of different types of pulp.

In all modicatons it will be apparent that the introduced fluid willforce circulation of the adjacent mass of chips and fiber, which inthemselves additionally tend to prevent the formation of a mat andprevent fouling the openings at the surface of the screen ring.

I claim:

1. A digester comprising a pressure vessel in which fibrous material anda cooking liquor may be placed for creating a fibrous pulp, acirculating system communicating with said pressure vessel and adaptedto withdraw cooking liquor from the pressure vessel and return itthereto, screen 'means comprisng a plurality of screen segments arrangedcircumferentially about the interior of said pressure vessel at anintermediate level therein and providing a barrier at the inlet of saidcirculating system to prevent the escape of more than a limited amountof fine, fibrous material from the pressure vessel, each of said screensegments having a plurality of rows of perforations therein, andscavenging means comprising a fluid conduit entering said pressurevessel and having a plurality of jet orifices closely adjacent saidscreen segments, each of said jet orifices being directed toward 'certanperforations of one of said screen segments whereby a fluid may beforced through said jet orifices to dislodge accumulations of fibrousmaterial from the surfaces of the screen segments.

2. In a digester including a vessel in which fibrous material may becooked in liquor to produce pulp, and liquor circulating meanscommunicating with a liquor discharge aperture below the surface of theliquor in the vessel during such cooking and a liquor feed aperture forsuch vessel spaced from the discharge aperture, for withdrawing cookingliquor from the body of cooking liquor in the vessel and returning it tosuch body, the combination of screen means located within the vessel andbelow the surface of the liquor therein during cooking and separatingsuch liquor discharge aperture from the portion of the vessel containingthe fibrous material and cooking liquor mixture to strain the liquorwithdrawn from the cooking liquor body into the circulating means, and ascavenging fluid conduit including, within such vessel, an internalconduit portion below the surface of the liquor in the vessel duringcooking, adjacent to said screen means and having a jet orifice directedgenerally toward said screen means and operable to project a jet foragitating the mass of fibre adjacent to said screen means and preventingclogging of such means by fibrous material.

3. In a digester including a vessel, the walls of which are curved, inwhich fibrous material may be cooked in liquor to produce pulp, andliquor circulating means communicating With a liquor discharge aperturebelow the surface of the liquor in the vessel during such cooking and aliquor feed aperture for such vessel spaced from the discharge aperture,for withdrawing cooking liquor from the body of cooking liquor in thevessel and returning it to such body, the combination of screen meanslocated within the vessel on a curved wall thereof below the surface ofthe liquor therein during cooking and separating such liquor dischargeaperture from the portion of the vessel containing the fibrous materialand cooking liquor mixture to strain the liquor withdrawn from thecooking liquor "body into the circulating means,

and a fluid conduit including, within such vessel, an internal conduitportion below the surface of the liquor in the vessel during cooking,adjacent to said screen means and having a jet orifice directed closealongsde and generally parallel to said screen means.

4. In a digester including a vessel in which fibrous material `may becooked in liquor to produce pulp, and liquor circulating meanscommunicating with a liquor discharge aperture below the surface of theliquor in the vessel during such cooking and a liquor feed aperture forsuch vessel spaced from the discharge aperture, for

withdrawing cooking liquor from the body of cooking liquor in the vesseland returning it to such body, the combination of screen means locatedwithin the vessel and below the surface of the liquor therein duringcooking and separating such liquor discharge aperture from the portionof the vessel containing the fibrous material and cooking liquor mixtureand having a multiplicity of perforations arranged in rows to strain theliquor withdrawn from the cooking liquor body into the circulatingmeans, and a scavenging fluid conduit including, within such vessel, aninternal conduit portion below the surface of the liquor in the vesselduring cooking, adjacent to said screen means and having a plurality ofnarrow jet orifices directed along and generally parallel to the rows ofscreen means perforations, respectively, and adjacent thereto andoperable to project jets for preventing clogging of said screen means byfibrous material.

5. In a digester, the combination defined in claim 4, wherein the screenmeans comprises a screen plate having a bulging side nearer thecenterline of the vessel, the internal conduit portion being locatedclosely adjacent to the lower edge of said screen plate and the jetorifices being directed upwardly along the respective rows ofperforations of said bulging screen plate side.

6. In a digester, the combination defined in claim 4, wherein the screenmeans compn'ses a screen plate having a bulging side nearer thecenterline of the vessel, the internal conduit portion being locatedclosely adjacent to the upper edge of said screen plate, and the jetorifices being directed downwardly along the respective rows ofperforations of said bulging screen plate side.

7. In a digester including a vessel, the walls of which are curved, inwhich fibrous material may be cooked in liquor to produce pulp, andliquor circulating means communicating with a liquor discharge aperturebelow the surface of the liquor in the vessel during such cooking and aliquor feed aperture for such vessel spaced from the discharge aperture,for withdrawing cooking liquor from the body of cooking liquor in thevessel and returning it to such body, the combination of a plurality ofscreen plates on one of the curved walls of the vessel below the surfaceof the liquor therein during cooking, spaced horizontally along suchwall and separating such liquor discharge aperture from the portion ofthe vessel containing the fibrous material and cooking liquor nu ture,and having a multiplicity of perforations to strain the liquor withdrawnfrom the cooking liquor body into the circulating means, and ascavenging fluid conduit ncluding, within such vessel, a conduit portiondisposed alongside one of said screen plates and having a plurality ofjet orifices therein directed alongside said one screen plate andoperable to project fluid jets therefrom for preventing clogging of saidscreen plateiby fibrous material.

8. In a digester including a vessel, the walls of which are curved, inwhich fibrous material may be cooked n liquor to produce pulp, andliquor circulatingmeans communicating with a liquor discharge aperturebelow the surface of the liquor in the vessel durng such cooking and aliquor feed aperture for such vessel spaced from the discharge aperturefor withdrawing cooking liquor from the body of cooking liquor in` thevessel and returning t to such body, the combination of a pluralty ofscreen plates projecting inward from a curved sde wall of the vesselbelow the surface of the liquor therein during cooking, spacedhorizontally along such wall and separating such liquor dischargeaperture from the portion of the vessel containing the fibrous materialand cooking liquor mixture, and having a multiplicity of perforationsarranged in horizontal rows to strain the liquor withdrawn from thecooking liquor body into the circulating means, and a scavenging fluidconduit including, within such vessel, a conduit portion disposedalongside one of said screen plates and having a plurality of narrowjets orifices therein directed along and generally parallel to the rowsof screen plate perforations, respectively, and adjacent thereto andoperable to project fluid jets therefrom for preventing clogging of saidscreen plate by fibrous material.

9. In a digester including a vessel, the walls of which are curved, withits axis extending substantially vertically in which fibrous materialmay be cooked in liquor to produce pulp, and liquor circulating meansdisposed externally of such vessel and communicatng with a liquordischarge aperture in a curved side wall of the vessel below the surfaceof the liquor in the vessel during such cooking and a liquor feedaperture in such vessel spaced from the discharge aperture forwithdrawing cooking liquor from the body of cooking liquor in the vesseland returning it to such body, the combination of screen means ofarcuate shape having its center substantially coinciding with the axisof the vessel, and a scavenging fluid conduit entering the vessel andincluding, within such vessel, an internal conduit portion of arcuateshape corresponding to the arcuate shape of said screen means, having aplurality of jet orifices spaced lengthwise of it and directed alongsideand generally parallel to said screen means and operable to project jetsfor preventing clogging of said screen means by fibrous material.

10. In a digester including a vessel, the walls of which are curved,with its axis extending substantially vertically in which fibrousmaterial may be cooked in liquor to produce pulp, and liquor circulatingmeans disposed externally of such vessel and communicating with a liquordischarge aperture in one of the curved walls of the vessel below thesurface of the liquor in the vessel during such cooking and a liquorfeed aperture in such vessel spaced from the discharge aperture, forwithdrawing cooking liquor from the body of cooking liquor in the vesseland returning it to such body, the combination of screen means ofannular shape having its center substantially coinciding with the axisof the vessel and the side of said screen means nearer the center of thevessel is convex in radial planes of the vessel, and a scavenging fluidconduit entering the vessel and including, within such vessel, aninternal conduit portion of annular shape corresponding to the annularshape of said screen means, having a plurality of jet orifices spacedcircumferentially of it and directed alongside and generally parallel tosaid screen means and operable to project jets for preventing cloggingof said screen means by fibrous material.

11. In a digester including a vessel in which fibrous material may becooked in liquor to produce pulp, and liquor circulating meanscommunicatingwith a liquor discharge aperture below the surface of theliquor in the vessel during such cooking and a liquor feed aperture forsuch vessel spaced from the discharge aperture, for withdrawing cookingliquor from the body of cooking liquor in the vessel and returning it tosuch body, the combination of screen means spaced inwardly from a wallof such vessel below the surface of the liquor therein during cookingand separating such liquor discharge aperture from the portion of thevessel containing the fibrous material and cooking liquor mixture, andhaving a multiplicity of perforations to strain the liquor Withdrawnfrom the cooking liquor body into the circulating means, and ascavenging fluid conduit including, within such vessel, an internalconduit portion located between said screen means and the adjacentportion of the vessel wall and having a jet orifice disposedsubstantially in contact with the adjacent side of said screen means fordirecting a jet of fluid from said orifice through a perforation of saidscreen means.

12. In the production of pulp, the method of cooking fibrous materialsin a body of cooking liquor in a vessel which comprises strainingcooking liquor through a screen beneath the surface of the body ofcooking liquor in the vessel and withdrawing such strained liquor fromsuch cooking liquor body, and, at a location beneath the surface of thebody of cooking liquor in the vessel and adjacent to such screen,directing a fluid jet to agitate the fibrous material adjacent to thescreen and prevent clogging of such screen by fibrous material to enablecontinued flow of liquor through the screen.

13. The method defined in claim 12, in which the liquor withdrawn fromthe body of cooking liquor in the vessel is returned to such body ofcooking liquor, and the fluid of the jet is supplemental to such liquorreturned to the body of cooking liquor.

14. The method defined in claim 12, in which the fluid of the jet isheated prior to being supplied to the vessel.

'15. The method defined in claim 12, in which the fluid of the jet iscooking liquor withdrawn through the screen and being returned to thebody of cooking liquor in the vessel.

16. The method defined in claim 12, in which the fluid of the jet issteam.

17. The method defined in claim 12, in which the fluid of the jet is aninert gas.

18. 'I'he method defined in claim 12, in which the fluid of the jet hasa pI-l value farther from pI-I 7 but at the same side of pH 7 as the pHvalue of the body of liquor in the vessel, and the supplemental fluidacts to alter the pH value of such body of liquor to a value fartherfrom pH 7.

19. In the production of pulp, the method of cooking fibrous materialsin a body of cooking liquor in a vessel which comprises strainingcooking liquor through a screen located beneath the surface of the bodyof cooking liquor in the vessel and, adjacent to such screen, projectinga jet of hot gas into the mass of fibrous material and thereby eflectingheating of the liquor and agitation of such fibrous material adjacent tothe screen.

20. In the production of pulp, the method of cooking fibrous materialsin a body of cooking liquor in a vessel which comprises strainingcooking liquor through a screen located beneath the surface of the bodyof cooking liquor in the vessel and, adjacent to such screen, projectinga jet of steam into the mass of fibrous material and thereby efiectingheating of the liquor and agitation of such fibrous material adjacent tothe screen.

References Cited in the file of this patent UNITED STATES PATENTS1,942,685 Christensen Jan. 9, 1934 2,072,776 Svensson Mar. 2, 19372,163,437 Richmond June 20, 1939 2,179,796 Morterud Nov. 14, 19392,608,91O McCrystle Sept. 2, 1952

20. IN THE PRODUCTION OF PULP, THE METHOD OF COOKING FIBROUS MATERIALSIN A BODY OF COOKING LIQUOR IN A VESSEL WHICH COMPRISES STRAININGCOOKING LIQUOR THROUGH A SCREEN LOCATED BENEATH THE SURFACE OF THE BODYOF COOKING LIQUOR IN THE VESSEL AND, ADJACENT TO SUCH SCREEN, PROJECTINGA JET OF STEAM INTO THE MASS OF FIBROUS MATERIAL AND THEREBY EFFECTINGHEATING OF THE LIQUOR AND AGITATION OF SUCH FIBROUS MATERIAL ADJACENT TOTHE SCREEN.